Devices, such as a scanner and a printer, output device values (like RGB values or CMYK values) which are values depending on the respective devices, or device-dependent values. To handle device values, a color conversion table (a device profile) for converting device values into device-independent colors is created, and the device values are converted into colorimetric values in a device-independent color space by using the device profile. As an example of a way to create a device profile, a description is now given of a way to create a scanner profile. To create a scanner profile, a color chart is output with a printer, and the color chart is measured with a scanner and a colorimeter. RGB values obtained by measurement of the color chart with the scanner and L*a*b* values in the CIE 1976 color space or XYZ values of the CIE 1931 color space obtained by measurement of the color chart with the colorimeter are associated with each other, whereby a scanner profile is created.
In such a scanner file, the correspondence between RGB values and colorimetric values can deteriorate with a change of an individual body or a component of a scanner, or a change of a paper type used for scanning. It can worsen the accuracy of estimation of colorimetric values (in other words, the accuracy of conversion of RGB values into colorimetric values) by using the scanner profile. In order to estimate colorimetric values with accuracy, there is a need to recreate a scanner profile for each scanner and each paper type or to create a correction LUT (look-up table) to be used for correction of the scanner profile.
As an example of a technique to recreate a scanner profile, there is a technique to use a commercial software program and recreate the scanner profile by obtaining a color chart reprinted with a printer and measuring the color chart with both a scanner and a colorimeter. As an example of a technique to correct a scanner profile in response to an occurrence of the fluctuation of scanner characteristics, Japanese Unexamined Patent Publication (JP-A) No. 2006-033572 discloses a technique to reduce a load of an image processing apparatus by causing a high-speed external device to calculate a scanner profile.
With regard to a technique to measure a color chart with both a scanner and a colorimeter, which is not a technique to correct a scanner profile, U.S. Pat. No. 7,505,173 discloses a method of using a printer equipped with a scanner and a spectrocolorimeter to create a tone curves of CMYK colors to be used for correcting colors output by the printer. As another example, JP-A No. 2015-226128, which corresponds to United States Patent Application Publication No. US2015/0350493A1, discloses the following image forming apparatus. The image forming apparatus uses a spectrocolorimeter and a line sensor to measure plural common color patches formed on a same surface of a same sheet, where the spectrocolorimeter is a device that measures only a partial region in a main scanning direction in the original, and the line sensor is a device that measures the entire region of an image formation width in the main scanning direction in the original. On the basis of measurement information given by measurement of the common color patches with both of the spectrocolorimeter and the line sensor, the image forming apparatus estimates, from measurement information obtained with the line sensor, values equivalent to measurement information to be obtained with the spectrocolorimeter.
To correct a scanner profile, the above-described technique using a commercial software program and the technique disclosed in JP-A No. 2006-033572 need a process to recreate the scanner profile from the beginning, and thus need sufficient time to correct the scanner profile. These techniques further need, at each time when a scanner profile is corrected, a process of outputting a color chart for creating a scanner profile, which may waste time and resources in printing color charts.
By using the techniques disclosed in U.S. Pat. No. 7,505,173 and JP-A No. 2015-226128 to establish a system which can measure patches of a color chart with both of an in-line scanner and an in-line colorimeter, a scanner profile can be corrected with RGB values and colorimetric values of the patches. However, those values obtained on creation of a scanner profile and those values obtained on correction of the scanner profile should not be compared simply with each other since the measurement conditions of the patches on creation of the scanner profile are different from the measurement conditions of the patches on correction of the scanner profile.
In concrete terms, on creation of a scanner profile, patches of small size are used in a color chart so that the color chart can provide detailed color information. On correction of a scanner profile, there is a need to use patches of increased size in a color chart so that an in-line colorimeter can obtain colorimetric values efficiently. On comparison between color measurement of a large-sized patch and that of a small-sized patch, they differ in the degree of the influence of reflected light (the level of flare) coming from patches surrounding the patch to be measured. Therefore, even when a correction LUT is created on the basis of measurement of large-sized patches and then is used to correct a scanner profile which was created on the basis of measurement of small-sized patches, such operations would correct a scanner profile inappropriately.